Exhaust gas purifying catalyst and exhaust gas purifying system

ABSTRACT

An exhaust gas purifying catalyst comprises a substrate, an inner catalytic layer coated on the substrate, an intermediate catalytic layer containing Pd laid over said inner catalytic layer, and an outer catalytic layer containing a NOx adsorption material laid over said intermediate catalytic layer. The Pd isolated from the NOx adsorption material is prevented from lowering its low temperature activity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exhaust gas purifyingcatalyst and an exhaust gas purifying system using the exhaust gaspurifying catalyst.

[0003] 2. Description of Related Art

[0004] There have been widely known three-way catalysts that performsoxidization of HC and CO and reduction of NOx simultaneously. Such athree-way catalyst often comprises an active metal such a noble metal asPd, Pt and Rh, a support material such as alumina that functions tostabilize the active metal and, as a result, to increase a surface areaof the active metal coming into contact with exhaust gases so as therebyto improve the catalytic conversion efficiency thereof, and an oxygenstorage material such as ceria (a supplemental catalyst). The three-waycatalyst shows poor catalytic performance at low exhaust gas temperatureand causes significant aggravation of NOx conversion efficiency at leanair-fuel ratios.

[0005] One of catalysts of the type containing an HC absorption materialand a NOx adsorption material is disclosed in Japanese Unexamined PatentPublication No. 2001-113173. This catalyst stores HC and NOx in anexhaust gas during cold engine operation immediately after an enginestart and releases and converts the adsorbed HC and NOx after activationof a catalytic metal. The catalyst comprises an inner catalytic layer ofHC absorption material that contains zeolite on a surface of a substrateand an outer catalytic layer of catalytic metal that contains a noblemetal such as Pd and a NOx adsorption material such as Ba. The NOxadsorption material is selected from a group of alkaline metals andalkaline earth metals. The catalyst contains the NOx adsorption materialbetween 60/40 and 99/1 in weight ratio.

[0006] Another catalyst of the type containing an HC absorption materialand a NOx adsorption material is disclosed in Japanese Unexamined PatentPublication No. 11-13462. This catalyst comprises an inner catalyticlayer of HC absorption material that contains zeolite on a surface of asubstrate and one or two outer catalytic layers of catalytic metal eachof which contains a noble metal such as Pd and is impregnated with asolution of barium nitrate.

[0007] As disclosed, for example, in Japanese Unexamined PatentPublication No. 9-79026, it has been known to dispose a NOx adsorptionmaterial and an HC absorption material coated on a single substratetogether with a three-way catalyst in an exhaust line.

[0008] The investigation of peculiarities of this type of catalysts thatwas conducted by the inventors of the present application showed that,although Pd in the catalyst inherently had catalytic activity on HC atcomparatively low temperatures, there were cases where the lowtemperature catalytic performance of Pd was lowered depending on thecatalytic composition when the catalyst was exposed to a hightemperature exhaust gas. It was found that these cases appeared in thecatalyst that contained an alkaline metal or an alkaline earth metal asthe NOx adsorption material. It was also found that the more the lowtemperature activity of Pd was deteriorated more as the amount of NOxadsorption material increased.

[0009]FIG. 8 shows light off temperatures (T50) regarding HC, CO and NOxconversion for various comparative catalysts containing differentamounts of NOx adsorption material after aging. The comparative catalystcomprises two catalytic layers, namely an inner catalytic layer coatedon a substrate and an outer catalytic layer coated over the innercatalytic layer. The inner catalytic layer has a catalytic componentconsisting of Ag and Bi supported on β-type zeolite. The outer catalyticlayer has a catalytic component consisting of Pt, Rh and Pd supported onalumina and ceria. The measurement of light off temperature was made onfour comparative catalysts having outer catalytic layers that contain noNOx adsorption material, 16 g/L of NOx adsorption material (10 g/L ofBa; 3 g/L of Sr and 3 g/L of Mg), 32 g/L of NOx adsorption material (20g/L of Ba; 6 g/L of Sr and 6 g/L of Mg) and 50 g/L of NOx adsorptionmaterial (30 g/L of Ba; 10 g/L of Sr and 10 g/L of Mg), respectively.Each comparative catalyst was aged at 80° C. for 24 hours in theatmosphere.

[0010] As demonstrated in FIG. 8, each of HC, CO and NOx shows a rise inlight off temperature that becomes greater as the amount of NOxadsorption material increases. In particular, each of HC and CO shows aprominent tendency to rise the light off temperature.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of the invention to provide an exhaustgas purifying catalyst containing both Pd and NOx adsorption materialthat prevents Pd from causing aggravation in low temperature HCpurification performance when the exhaust gas purifying catalyst isexposed to a high temperature exhaust gas.

[0012] It is another object of the invention to provide an exhaust gaspurifying catalyst containing both Pd and NOx adsorption material thatprevents Pd from causing aggravation of its low temperature activitymore as the NOx adsorption material is increased in amount.

[0013] The present invention was accomplished on the basis of therevelation that the functional aggravation of an exhaust gas purifyingcatalyst containing both Pd and a NOx adsorption material as an alkalinemetal or an alkaline earth metal is prevented by isolating Pd from theNOx adsorption material in the exhaust gas purifying catalyst.

[0014] According to an aspect of the present invention, the exhaust gaspurifying catalyst contains at least a metal, selected from a group ofalkaline metals and alkaline earth metals; and Pd, such that the Pd ispartly isolated from the metal so as to be not affected by said metalelectrically nor chemically. Specifically, the exhaust gas purifyingcatalyst comprises an outer catalytic layer containing a metal and aninner catalytic layer containing Pd. These Pd and metal are preventedfrom electrical and chemical interaction with each other, so that theisolated Pd is prevented from causing aggravation in low temperatureactivity.

[0015] The exhaust gas purifying catalyst may further comprise aninnermost layer of zeolite that is operative to adsorb HC in an exhaustgas while the exhaust gas purifying catalyst remains low in temperatureand to release the adsorbed HC into the exhaust gas when the exhaust gaspurifying catalyst falls in temperature. The zeolite adsorbs HC in anexhaust gas while the exhaust gas purifying catalyst is at a lowtemperature, so that HC is prevented from emitting into the atmosphereis prevented. Since the metal, an alkaline metal or an alkaline earthmetal, does not affect the low temperature activity of Pd, HC releasedfrom the zeolite catalytic layer is reliably oxidized, and so purified,by Pd.

[0016] It has been known in the art that, although Pd is advantageous tothe low temperature HC conversion, it is easy to cause thermaldeterioration and to be poisoned with lead and sulfur.Contradistinctively, the exhaust gas purifying catalyst of the presentinvention in which the inner catalytic layer containing Pd is covered bythe outer catalytic layer containing the metal, so that the outercatalytic layer functions as a functional barrier, preventing the Pdfrom thermal deterioration and lead and sulfur poisoning. Therefore, Onthis account, the low temperature activity of the exhaust gas purifyingcatalyst is ensured.

[0017] The exhaust gas purifying catalyst may preferably contain themetal more than 15 g per one liter of a substrate on which the exhaustgas purifying catalyst is formed. Even though the exhaust gas purifyingcatalyst contains a large amount of the metal, an alkaline metal or analkaline earth metal, the exhaust gas purifying catalyst prevents the Pdfrom lowering its lower temperature activity. In the case where analkaline metal or an alkaline earth metal is employed as a NOxadsorption material, it is preferred for the exhaust gas purifyingcatalyst to contain the metal 30 g, but less than 59 g, per one liter ofthe substrate.

[0018] According to another aspect of the present invention, an exhaustgas purifying system comprises a catalyst that is disposed in theexhaust line and comprises at least a metal selected from a group ofalkaline metals and alkaline earth metals, a NOx adsorption materialoperative to adsorb NOx in the exhaust gas while the exhaust gas has acomparatively high oxygen concentration, to release the adsorbed NOxinto the exhaust gag, and to adsorb SOx in the exhaust gas as saidexhaust gas lowers its oxygen concentration, and Pd, and temperaturecontrol means for raising a temperature of the catalyst so as thereby tocause the NOx adsorption material to release the adsorbed SOx into theexhaust gas. In the catalyst, the Pd is partly isolated from the NOxadsorption material so as to be not affected by said NOx adsorptionmaterial electrically nor chemically.

[0019] The catalyst containing a NOx adsorption material encounters theproblem of sulfur poisoning that refers to a loss of the NOx adsorptionfunction due to a salt formed in the form of an oxide of sulfurresulting from adsorption of sulfur in an exhaust gas. In the event ofan occurrence of this problem, although the oxide of sulfur can bereleased from the NOx adsorption material by raising the catalytictemperature, for example to 400° C., the NOx adsorption materialencourages the Pd in deteriorating its low temperature activity at a sohigh catalytic temperature, so as to cause the Pd to aggravate its lowtemperature activity significantly.

[0020] Contradistinctively, since the catalyst used in the exhaust gaspurifying system of the present invention in which the Pd is at leastpartly isolated from the NOx adsorption material so as to be notaffected by the NOx adsorption material electrically nor chemically, thePd is prevented from being encouraged in deteriorating its lowtemperature activity by the NOx adsorption material when the catalytictemperature is raised. The raise in exhaust gas temperature may beperformed by making an air-fuel ratio rich, by retarding an ignitiontiming or a fuel injection timing, or by supplying a secondary gas intoan exhaust gas stream so as to assist oxidative reaction of HC and COwith a result of providing an increase in reaction heat.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above and other objects and features of the present inventionwill be understood from the following description of a specificembodiment thereof when considering in conjunction with the accompanyingdrawings, in which:

[0022]FIG. 1 is a schematic view of an engine equipped with an exhaustgas purifying system of the present invention;

[0023]FIG. 2 is a cross-sectional view of an exhaust gas purifyingcatalyst according to an embodiment of the present invention;

[0024]FIG. 3 is graphic representation showing HC absorption ratios, HCoxidation ratios, total HC conversion ratio and NOx conversion ratio ofthe exhaust gas purifying catalyst shown in FIG. 2 before and afteraging;

[0025]FIG. 4 is a cross-sectional view of a comparative exhaust gaspurifying catalyst;

[0026]FIG. 5 is graphic representation showing HC absorption ratios, HCoxidation ratios, total HC conversion ratio and NOx conversion ratio ofthe exhaust gas purifying catalysts shown in FIGS. 2 and 4 after aging;

[0027]FIG. 6 is graphic representation showing the relationship betweeninlet temperature and HC conversion ratio of the exhaust gas purifyingcatalysts shown in FIGS. 2 and 4 after aging;

[0028]FIG. 7 is a flowchart illustrating a sequence routine of fuelinjection control; and

[0029]FIG. 8 is graphic representation showing the relationship betweenlight off temperature and amount of NOx adsorption material of anexhaust gas purifying catalyst after aging for HC, CO and NOx.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT

[0030] The term “light off temperature (T50)” as used herein shall meanand refer to an inlet gas temperature at which the conversion efficiencyof a catalyst amounts to 50%. Further, the term “exhaust gas of anair-fuel ratio (A/F) of X” as used herein shall mean and refer to anexhaust gas produced resulting from combustion of an air-fuel mixture ofan air-fuel ratio (A/F) of X.

[0031] Referring to the drawings in detail and, in particular, to FIG. 1schematically showing an engine 1 equipped with an exhaust gas purifyingsystem of the present invention, the engine 1, that is of aspark-ignition type, comprises a plurality of cylinders 2 (only one ofwhich is shown) in each of which a combustion chamber 4 is formed, afuel injector 3 operative to spray fuel directly into the combustionchamber 4, a spark plug 5. Air is introduced into the engine through anintake passage 6 and an exhaust gas is discharged from the engine 1through an exhaust passage 7 equipped with an exhaust gas purifyingcatalyst 8 therein.

[0032]FIG. 2 shows a cross-section of the exhaust gas purifying catalyst8. As shown, the exhaust gas purifying catalyst 8 comprises a substrate11 such as a cordierite honeycomb bed, an inner catalytic layer 12coated on the substrate 11, an intermediate catalytic layer 13 laid overthe inner catalytic layer 12 and an outer catalytic layer 14 laid overthe intermediate catalytic layer 13. The inner catalytic layer 12contains Ag and Bi in addition to zeolite, and a binder. Theintermediate catalytic layer 13 contains a catalytic component thatcomprises Pd supported on alumina and ceria, and a binder. The outercatalytic layer 14 comprises a catalytic component that contains Pt, Rh,Ba, Sr and Mg supported on alumina and ceria, and a binder. The zeoliteof the exhaust gas purifying catalyst 8, that functions as an HCabsorption material, is of a β-type having a salic ratio of 300.

[0033] The catalytic component comprising Pt and Rh supported on aluminasuch as γ-alumina powder and ceria functions as a three-way catalyst.Ba, Sr and Mg in the exhaust gas purifying catalyst 8 adsorb NOx in anexhaust gas when the exhaust gas has an oxygen concentration higherthan, for example, 4% that is represented by an air-fuel ratio greaterthan 16 and release the adsorbed NOx into the exhaust gas when theexhaust gas lowers the oxygen concentration below, for example, 2%. Thealumina is used in the powder form. As the ceria, that indicates oxidescontaining a ceria component and functions as an oxygen storagematerial, one of Ce.Pr composite oxides (Ce₀ ₉Pr₀ ₁O₂) is employed inthis embodiment. In order to put a restraint on thermal deterioration ofthe alumina, a small amount, for example 5%, of La is added into thealumina powder. As the binder of each of the inner catalytic layer 12and the intermediate catalytic layer 13, an alumina binder is employeda. On the other hand, as the binder of the outer catalytic layer 14, abasic binder such as zirconia binder. Seen in that sense, the exhaustgas purifying catalyst 8 can be called a lean NOx adsorption catalysthaving an HC trapping function.

[0034] A sample exhaust gas purifying catalyst 8 was produced in thefollowing process. First of all, catalytic powder A and B were prepared.The catalytic powder A was made up by mixing active alumina powder,ceria, palladium nitrate and water all together and calcinating themixture at 500° C. after drying. The catalytic powder B was made up bymixing active alumina powder, ceria, barium acetate, strontium acetate,magnesium acetate, dinitro-diamine platinum nitrate, rhodium nitrate andwater all together, and calcinating the mixture at 500° C. after drying.

[0035] The inner catalytic layer 12 was formed by coating a given amountof a slurry of a mixture of zeolite, hydrated alumina (alumina binder).The mixture slurry was prepared by mixing zeolite and hydrated aluminawith water and stirring the mixture. The honeycomb substrate 11 wasdipped into and drawn out from the mixture slurry so as to form a slurrylayer, and then the slurry layer was exposed to air blows so as toremove an excessive part of the mixture slurry. This process wasrepeated until a layer consisting of the given amount of the mixtureslurry is formed. The eventual layer was finished by calcinations at500° C. after drying. The intermediate catalytic layer 13 was formedover the inner catalytic layer 12 in the same process as the innercatalytic layer 12 using the catalytic powder A in place of zeolite. Theinner and intermediate catalytic layers 12 and 13 were impregnated witha mixed solution of silver nitrate and bismuth acetate and thencalcinated at 500° C. after drying. Thereafter, the outer catalyticlayer 14 was formed over the intermediate catalytic layer 13 in the sameprocess as the inner catalytic layer 12 using the catalytic powder B anda zirconia binder in place of zeolite and an alumina binder,respectively.

[0036] The exhaust gas purifying catalyst 8 was adjusted so as to havethe following quantitative formula: Inner catalytic layer 12 β-typezeolite: 100 g/L Intermediate catalytic layer 13 Pd: 2.0 g/L; Alumina:30 g/L; Ceria: 10 g/L Outer catalytic layer 14 Pt: 3.5 g/L; Rh: 0.3 g/L;Ba: 30 g/L; Sr: 10 g/L; Mg: 10 g/L; Alumina: 100 g/L; Ceria: 100 g/LImpregnated constituent Ag: 10 g/L Bi: 0.5 g/L

[0037] The inner and outer catalytic layers 12 and 14 were adjusted sothat the inner catalytic layer 12 contains a total amount of Ba, Sr andMg less than 1% of the total amount of those of the outer catalyticlayer 14. Each of the respective catalytic layers 12, 13 and 14contained impurities less than 1%.

[0038] A rig test was carried out in order to evaluate catalyticperformance—HC purification efficiency (HC absorption ratio, HCoxidation ratio and HC conversion efficiency) and lean NOx purificationefficiency—of the sample exhaust gas purifying catalyst 8 before andafter aging. The aging of the sample exhaust gas purifying catalyst 8was made in the atmosphere at 800° C. for 24 hours. The evaluation of HCpurification efficiency was performed in the test mode consisting ofraising the inlet gas temperature of an N₂ gas streaming at a spatialvelocity of 25000h⁻¹ in which the sample exhaust gas purifying catalyst8 was disposed to 80° C. (first step); supplying HC, NO and O₂ gasesinto the stream of N₂ gas for two minutes keeping the inlet gastemperature so that the HC, NO and O₂ contents amount to 1500 ppmC, 100ppmC and 1.0%, respectively, (second step); and subsequently raising theinlet gas temperature from 80° C. to 400° C. at a rate of 30° C./min.after interrupting the supply of HC gas (third step). The N₂ gas wasstreamed at a spatial velocity of 25000h⁻¹.

[0039] HC absorption ratio was determined on the basis of inlet andoutlet HC concentrations of the N₂ gas stream for the period of twominutes during the second step. The HC oxidation ratio, that is theratio of oxidized HC relative to absorbed HC, was determined on thebasis of an absorbed amount of HC in the sample exhaust gas purifyingcatalyst 8 for the period of two minutes during the second step and theoutlet HC concentration during the third step. The HC conversionefficiency was defined by the product of HC absorption ratio and HCoxidation ratio. The lean NOx purification efficiency was defined as aNOx conversion efficiency for a period of 60 seconds from a point oftime at which a simulated exhaust gas is changed over in gas compositionto a specific gas composition A after repeating exposure of the sampleexhaust gas purifying catalyst 8 to a simulated exhaust gas having thegas composition A for a period of 60 seconds and subsequently to asimulated exhaust gas having a specific gas composition B for a periodof 60 seconds five times. The simulated exhaust gas was maintained at35° C. and streamed at a spatial velocity of 25000h⁻¹. The table I showsthe gas composition A for a simulated exhaust gas of a lean air-fuelratio (A/F) of 22 and the gas composition B for a simulated exhaust gasof a rich air-fuel ratio (A/F) of 14.5. TABLE I Gas composition A Gascomposition B Lean (A/F = 22) Rich (A/F = 14.5) HC(C₃H₆) 1333 ppm  1333ppm  NO 260 ppm 260 ppm CO 0.16% 0.16% CO₂ 9.75% 9.75% H₂ 650 ppm 650ppm O₂ 7% 0.5% N₂ Reminder Remainder

[0040] The evaluation result is shown in FIG. 3. As demonstrated in FIG.3, the HC oxidation ratio shows a deterioration due to aging but not solarge. This event will be described in detail later in connection with arig test of a comparative exhaust gas purifying catalyst. On the otherhand, the HC absorption ratio shows deterioration a little due to aging.The lean NOx purification efficiency shows deterioration a little due toaging. The fact that deterioration of the HC absorption ratio is quite alittle indicates that the β-type zeolite in the inner catalytic layer 12causes almost no functional defect due to aging. This event proves thatthe β-type zeolite causes almost no decrease in specific surface area.

[0041] Bi impregnated in the intermediate catalytic layer 13, thatexists an accessible atom to a Pd atom, prevents Pd from reacting on Agand thereby producing a Pd—Ag alloy, in other words, prevents Pd fromcausing a deterioration in low temperature activity on HC conversion orprevents a decrease in the amount of Ag that effectively functions inthe improvement of HC absorption performance.

[0042]FIG. 4 shows a comparative exhaust gas purifying catalyst 8C thatcomprises an inner catalytic layer 12C coated on a substrate 11 such asa cordierite honeycomb bed and an outer catalytic layer 14C directlylaid over the inner catalytic layer 12C. The inner catalytic layer 12Cis identical in constituent with and formed in the same process as theinner catalytic layer 12 of the sample exhaust gas purifying catalyst 8.The outer catalytic layer 14C comprises a catalytic component thatcontains Pt, Rh, Pd, Ba, Sr and Mg supported on alumina and ceria, and abinder. That is, the comparative exhaust gas purifying catalyst 8C isdifferent from the sample exhaust gas purifying catalyst 8 in that nointermediate catalytic layer is formed between the inner and outercatalytic layers 12C and 14C, but is there Pd additionally contained inthe outer catalytic layer 14C.

[0043] The comparative exhaust gas purifying catalyst 8C was produced inthe following process. In the first place, catalytic powder C wasprepared. The catalytic powder C was made up by mixing active aluminapowder, ceria, barium acetate, strontium acetate, magnesium acetate,dinitro-diamine platinum nitrate, rhodium nitrate, palladium nitrate andwater all together and calcinating the mixture at 500° C. after drying.

[0044] The inner catalytic layer 12C was formed in the same process asthat of the sample exhaust gas purifying catalyst 8. The inner catalyticlayer 12C was subsequently impregnated with a mixed solution of silvernitrate and bismuth acetate and then calcinated at 500° C. after drying.Thereafter, the outer catalytic layer 14C was formed over the innercatalytic layer 12C in the same process as that of the sample exhaustgas purifying catalyst 8.

[0045] The exhaust gas purifying catalyst 8C was adjusted so as to havethe following quantitative formula: Inner catalytic layer 12C β-typezeolite: 100 g/L Outer catalytic layer 14C Pt: 3.5 g/L; Rh: 0.3 g/L; Pd:2.0 g/L; Ba: 30 g/L; Sr: 10 g/L; Mg: 10 g/L; Mg: 10 g/L; Alumina: 100g/L; Ceria: 100 g/L Impregnated constituent Ag: 10 g/L Bi: 0.5 g/L

[0046] The inner and outer catalytic layers 12C and 14C were adjusted sothat the inner catalytic layer 12C contains a total amount of Ba, Sr andMg less than 1% of the total amount of those of the outer catalyticlayer 14. Each of the respective catalytic layers 12C and 14C containedimpurities less than 1%.

[0047] A rig test was carried out in order to make comparativeevaluations of the catalytic performance—HC purification efficiency andlean NOx purification efficiency—between the sample and comparativeexhaust gas purifying catalysts 8 and 8C after aging in the same testmode as described in connection with the evaluation rig test of thesample exhaust gas purifying catalyst 8. The evaluation result is shownin FIG. 5.

[0048] As demonstrated in FIG. 5, there is almost no difference in thelean NOx conversion efficiency between the sample and comparativeexhaust gas purifying catalysts 8 and 8C. The sample exhaust gaspurifying catalyst 8 shows an HC absorption ratio lower than thecomparative exhaust gas purifying catalyst 8C and, however, an HCoxidation ratio higher than the comparative exhaust gas purifyingcatalyst 8C. As a whole, the sample exhaust gas purifying catalyst 8 hasan HC conversion ratio higher than the comparative exhaust gas purifyingcatalyst 8C.

[0049] It is conceivable that the high HC oxidation ratio of the sampleexhaust gas purifying catalyst 8 is due to the conformation of Pd in theintermediate catalytic layer 13 isolated from Ba, Sr and Mg functioningas NOx adsorption elements in the outer catalytic layer 14. That is,because Pd is higher in low temperature activity on HC conversion due tooxidation, the HC oxidation ratio is greatly influenced by how the Pd isactive. As for the sample exhaust gas purifying catalyst 8, the Pd inthe intermediate catalytic layer 13 is hardly affected electrically andchemically by the NOx adsorption elements, so that the Pd does not causea significant deterioration in the low temperature activity due toaging. That is, it is conceived that the NOx adsorption elements do notencourage the Pd in deteriorating its low temperature activity.

[0050] Rig tests were carried out to measure HC conversion ratios of thesample and comparative exhaust gas purifying catalysts 8 and 8C in orderto evaluate light off temperatures (T50) regarding HC conversion.Measurements of HC conversion ratio were made on each catalyst afteraging as the inlet gas temperature was gradually raised. Aging of thecatalyst was performed in the same condition as previously described. Asimulated exhaust gas that was used in the rig test was of an air-fuelratio (A/F) of 14.7±0.9. That is, while a main simulated exhaust gas ofan air-fuel ratio (A/F) of 14.7 was stationarily streamed at a spatialvelocity of 25000h⁻¹, a specified amount of modifying gas was spoutedinto the stationary main exhaust gas stream on a cycle of 1 Hz so as toforce the air-fuel ratio to pulsate between 14.7±0.9.

[0051] The table II shows the gas composition of the main simulatedexhaust gas of an air-fuel ratio (A/F) of 14.7. TABLE II CO₂ O₂ CO H₂C₃H₆(HC) NO N₂ 13.9% 0.6% 0.6% 0.2% 0.056% 0.1% Remainder

[0052] A gas of O₂ was employed for the modifying gas in order to forcethe air-fuel ratio to vary to a lean side to 15.6. On the other hand, amixture gas of H₂ and CO was employed for the modifying gas in order toforce the air-fuel ratio to vary to a rich side to 13.8.

[0053] The evaluation result is shown in FIG. 6. As demonstrated in FIG.6, the sample exhaust gas purifying catalyst 8 in which Pd is isolatedfrom Ba, Sr and Mg is superior in low temperature activity to thecomparative exhaust gas purifying catalyst 8C in which Pd coexists withBa, Sr and Mg. Further, the sample exhaust gas purifying catalyst 8 isimproved in light off temperature (T50) by approximately 27° C. ascompared with the comparative exhaust gas purifying catalyst 8C. Thisevent proves that the isolation of Pd from a NOx adsorption material isadvantageous to preventing an exhaust gas purifying catalyst fromcausing a deterioration of low temperature activity.

[0054] The exhaust gas purifying catalyst according to the embodiment ofthe present invention contains a NOx adsorption material such as Ba andadsorbs NOx in exhaust gases while an air-fuel ratio remains lean.Accordingly, when there is an increase in the amount of adsorbed NOx, itis necessary to cause reduction purification of NOx by means of makingan air-fuel ratio rich, i.e. by means of lowering an oxygenconcentration of exhaust gas, so as to release the adsorbed NOx. Inaddition, when the amount of NOx adsorption material poisoned withsulfur becomes large, it is necessary to revitalize the exhaust gaspurifying catalyst by means of raising a catalytic temperature.

[0055] The following description will be directed to fuel injectioncontrol for NOx release and revitalization of sulfur poisoned catalyst.In the fuel injection control, an air-fuel ratio is made rich in orderto cause a rise in catalytic temperature.

[0056]FIG. 7 shows a flowchart illustrating a sequence routine of thefuel injection control. When the sequence logic commences and controlproceeds to a function block at step S1 where various data are input.The data includes at least an amount of intake air, an engine speed, anaccelerator position or engine load and a catalytic temperature. Afterdetermining a basic amount of fuel injection Qpb that meets a targetair-fuel ratio determined on the basis of the data according to anengine operating condition at step S2, an amount of NOx adsorption NOeand an amount of SOx (oxides of sulfur) adsorption are estimated asintegrated values at steps S3 and S4, respectively. In this instance,when the air-fuel ratio remains lean, i.e. an excess air ratio λ isgreater than 1, this estimate is performed with respect to an engineoperating condition (engine speeds and engine loads) by reference to amap that defines amounts of NOx or SOx adsorption according to engineoperating conditions by way of experiment. On the other hand, when theair-fuel ratio remains rich, i.e. an excess air ratio λ is equal to orless than 1, the estimate is performed by gradually reducing theintegrated amount of NOx or SOx adsorption by a diminution constant thatis changed larger as the excess air ratio λ becomes lower and as aninterval for which the air-fuel ratio remains rich becomes longer.

[0057] Thereafter, a determination is made at step S5 as to whether theestimated amount of NOx adsorption NOe is larger than a predeterminedthreshold amount of NOx adsorption NOo. The threshold amount of NOxadsorption NOo refers to an amount of NOx adsorption that is conceivedto be as large as the exhaust gas purifying catalyst 8 involves NOxrelease control, in other words, the catlyst lowers its NOx adsorptivepower. When the estimated amount of NOx adsorption NOe is larger thanthe predetermined threshold amount of NOx adsorption NOo, this indicatesthat the sulfur poisoning of NOx adsorption material is as serious asthe exhaust gas purifying catalyst 8 needs to be revitalized, then,another determination is made at step S6 as to whether the lastattribute TN_(n−1) that a counter shows is zero. The attribute TNrepresents a period of time for which the fuel injection control keepsup control of increasing the amount of fuel injection. When the lastattribute TN_(n−1) is zero, after establishing a threshold time TNo atstep S7, after establishing a threshold time TSo at step S7, the currentattribute TN_(n) is changed by an increment of one at step S8. On theother hand, when the last attribute TN_(n−1) is not zero, the currentattribute TN_(n) is changed by an increment of one at step S8 withoutestablishing a threshold time TNo. In this instance, the threshold timeTNo is set to an appropriate value, for example between 0.5 and 5seconds in actual time, according to the current attribute TN_(n).Specifically, the threshold time TNo is small while the catalytictemperature is in a range, for example, between 200 and 400° C. whereNOx is easily released and is, on the other hand, large when thecatalytic temperature is out of that temperature range.

[0058] Subsequently to changing the current attribute TN_(n) by anincrement of one at step S8, a determination is made at step S9 as towhether the current attribute TN_(n) is larger than the threshold timeTNo. When the current attribute TN_(n) is equal to or smaller than thethreshold time TNo, the basic amount of fuel injection Qpb is replacedwith an amount of fuel injection Qpλ for enrichment at step S10.Thereafter, after substituting the basic amount of fuel injection Qpb(i.e. Qpλ) for an actual amount of fuel injection Qp at step S11, fuelinjection is controlled so as to spray the actual amount of fuelinjection Qp at step S12. On the other hand, when the current attributeTN_(n) is larger than the threshold time TNo at step S9, after resettingthe attribute TN_(n) and the estimated amount of NOx adsorption NOe atstep S13, the basic amount of fuel injection Qpb is substituted for anactual amount of fuel injection Qp at step S11 and then, fuel injectionis controlled so as to spray the actual amount of fuel injection Qp atstep S12.

[0059] On the other hand, when the estimated amount of NOx adsorptionNOe is equal to or smaller than the predetermined threshold amount ofNOx adsorption NOo, this indicates that the sulfur poisoning of NOxadsorption material is not serious, then, another determination is madeat step S14 as to whether the counter is still counting, i.e. whether itis still in the process of the control of increasing control ofincreasing the amount of fuel injection. When it is still in the processof the fuel increasing control, then, the sequence logic jumps to thecontrol of releasing NOx through steps S8 to S12. On the other hand,when it is out of the process of the fuel increasing control, then, adetermination is made at step S15 as to whether the estimated amount ofSOx adsorption SOe is larger than a predetermined threshold amount ofSOx adsorption SOo. The threshold amount of SOx adsorption SOo refers toan amount of SOx adsorption that is conceived to be poisoned with SOx aslarge as the exhaust gas purifying catalyst 8 involves SOx releasecontrol. When the estimated amount of SOx adsorption SOe is still belowthe predetermined threshold amount of SOx adsorption SOo, fuel injectionis performed so as to splay the basic amount of fuel Qpb through stepsS11 and S12.

[0060] On the other hand, when the estimated amount of SOx adsorptionSOe is above the predetermined threshold amount of SOx adsorption SOo atstep S15, another determination is made at step S16 as to whether thelast attribute TS_(n−1) that a counter shows is zero. The attribute TSrepresents a period of time for which the fuel injection control keepsup control of increasing the amount of fuel injection. When the lastattribute TS_(n−1) is zero, after establishing a threshold time TSo atstep S17, the current attribute TN_(n) is changed by an increment of oneat step S18. On the other hand, when the last attribute TS_(n−1) is notzero, the current attribute TS_(n) is changed by an increment of one atstep S18 without establishing a threshold time TSo. In this instance,the threshold time TSo is set to an appropriate value in a rangebetween, for example, 1 and 10 minutes in actual time according to acurrent catalytic temperature while the catalytic temperature is higherthan a specific temperature of, for example, 400° C. Specifically, thethreshold time TSo is made as the catalytic temperature becomes higher.

[0061] Subsequently, a determination is made at step S19 as to whetherthe current attribute TN_(n) is larger than the threshold time TSo. Whenthe current attribute TS_(n) is equal to or smaller than the thresholdtime TSo, the sequence logic jumps to the function at step S10 so as toreplace the basic amount of fuel injection Qpb with an amount of fuelinjection Qpλ for enrichment. Thereafter, after substituting the basicamount of fuel injection Qpb (i.e. Qpλ) for an actual amount of fuelinjection Qp at step S11, fuel injection is controlled so as to spraythe actual amount of fuel injection Qp at step S12. On the other hand,when the current attribute TS_(n) is larger than the threshold time TSoat step S19, after resetting the attribute TS_(n) the estimated amountof NOx adsorption NOe and the estimated amount of SOx adsorption SOe atstep S20, the basic amount of fuel injection Qpb is substituted for anactual amount of fuel injection Qp at step S11 and then, fuel injectionis controlled so as to spray the actual amount of fuel injection Qp atstep S12.

[0062] As described above, when the amounts of adsorbed NOx and adsorbedSOx increase, the air-fuel ratio is made rich by increasing the amountof fuel injection even while the engine is operated with a lean air-fuelratio, so as to lower the oxygen concentration of exhaust gas. As aresult, the exhaust gas purifying catalyst 8 releases NOx from the NOxadsorption material and then reduces the NOx with the catalytic novelmetals in the outer catalytic layer 14 thereof. Further, when theexhaust gas purifying catalyst 8 is poisoned with sulfur to asignificantly increased degree, the air-fuel ratio is made rich byincreasing the amount of fuel injection even while the engine isoperated with a lean air-fuel ratio. As a result, since, while theexhaust gas raises its temperature, the exhaust gas purifying catalyst 8is made active in oxidation reaction, the exhaust gas purifying catalyst8 raises its own temperature, resulting in releasing SOx from the NOxadsorption material. This leads to revitalization of the NOx adsorptionmaterial.

[0063] When the NOx adsorption material is seriously poisoned withsulfur, the exhaust gas temperature may be raised by retarding anignition timing in addition to making an air-fuel ratio rich. Further,the catalytic temperature may be further raised by supplying secondaryair into an exhaust gas stream upstream from the exhaust gas purifyingcatalyst 8 so as to assist the exhaust gas purifying catalyst 8 incausing oxidative reaction.

[0064] It is to be understood that although the present invention hasbeen described with regard to preferred embodiments thereof, variousother embodiments and variants may occur to those skilled in the art,which are within the scope and spirit of the invention, and such otherembodiments and variants are intended to be covered by the followingclaims.

What is claimed is:
 1. An exhaust gas purifying catalyst comprising atleast a metal selected from a group of alkaline metals and alkalineearth metals; and Pd, wherein said Pd is partly isolated from said metalso as to be neither electrically nor chemically affected by said metal.2. An exhaust gas purifying catalyst as defined in claim 1, comprisingtwo different catalytic layers formed on a substrate so as to be laidover each other, said two different catalytic layers containing said Pdand said metal separately.
 3. An exhaust gas purifying catalyst asdefined in claim 2, and further comprising a layer of zeolite laid undersaid two different catalytic layers, said layer of zeolite absorbing HCin an exhaust gas while said exhaust gas purifying catalyst remains lowin temperature and releasing HC into said exhaust gas when said exhaustgas purifying catalyst falls in temperature.
 4. An exhaust gas purifyingcatalyst as defined in claim 3, wherein outer one of said two differentcatalytic layer contains said metal and inner one of said two differentcatalytic layer contains said Pd.
 5. An exhaust gas purifying catalystas defined in claim 1, wherein exhaust gas purifying catalyst containssaid metal more than 15 g per one liter of said substrate.
 6. An exhaustgas purifying system for purifying exhaust gases discharged into anexhaust line from an engine, said exhaust gas purifying systemcomprising: a catalyst disposed in said exhaust line, said catalystcomprising at least a metal selected from a group of alkaline metals andalkaline earth metals, a NOx adsorption material operative to adsorb NOxin said exhaust gas while said exhaust gas has a comparatively highoxygen concentration and release said NOx adsorbed therein and adsorbsSOx as said exhaust gas lowers its oxygen concentration, and Pd; andtemperature control means for raising a temperature of said catalyst soas thereby to cause said NOx adsorption material to release said SOxadsorbed therein; wherein said Pd is partly isolated from said NOxadsorption material so as to be neither electrically nor chemicallyaffected by said NOx adsorption material.